Tumor growth and viability can be monitored by measuring the rate of glucose metabolism in the tumor, using fluorodeoxyglucose (FDG) positron emission tomography (PET). The goal of this imaging physics project is to assess various quantitative measures of tumor FDG uptake as indices of tumor response to therapy. The approach taken was to (1) develop and implement a model of glucose metabolism in normal and diseased liver; (2) investigate the errors associated with improper attenuation correction in studies of the abdomen, especially near the lung/liver tissue interface; and (3) compare the FDG-based tumor indices of tumor response to therapy with magnetic resonance imaging (MRI)-based measures.The first goal has nearly been achieved, although more testing is still necessary. Additional work is needed to be able to apply the models on a pixel-by-pixel basis. The second goal is completed, and a description of this work (combined with how such errors also affect cardiac studies) has recently been published in the Journal of Nuclear Medicine. Work on the third goal has resulted in a published abstract and an oral presentation at the Society of Nuclear Medicine. The technique is now ready for clinical testing. The method has been expanded to breast imaging. Using prone imaging, and a specially designed imaging mattress, breast tumor FDG uptake may be superimposed on the morphological data from PET or MRI. Ten subjects with breast tumors have been studied to date, including two both before and after surgery. A description of some of the practical issues associated with prone breast imaging has been presented at the Society of Nuclear Medicine.